An example of a portion of a prior art HDTV system 21 is depicted in FIG. 1. In such a system, a terrestrial analog broadcast signal 1 is forwarded to an input network or front end that includes an RF tuning circuit 14 and an intermediate frequency processor 16 including a double conversion tuner for producing an IF passband output signal 2. The broadcast signal 1 is a carrier suppressed eight bit vestigial sideband (VSB) modulated signal as specified by the Grand Alliance for HDTV standards. Such a VSB signal is represented by a one dimensional data symbol constellation where only one axis contains data to be recovered by the receiver 21. The passband IF output signal 2 generated by IF unit 316 is converted to an oversampled digital symbol datastream by an analog to digital converter (ADC) 19. The output oversampled digital datastream 3 is demodulated to baseband by a digital demodulator and carrier recovery network 22.
The recovery of data from modulated signals conveying digital information in symbol form usually requires that three functions be performed by receiver 21. First is timing recovery for symbol synchronization, second is carrier recovery (frequency demodulation to baseband), and finally channel equalization. Timing recovery is a process by which a receiver clock (timebase) is synchronized to a transmitter clock. This permits a received signal to be sampled at optimum points in time to reduce slicing or truncation errors associated with decision directed processing of received symbol values. Adaptive channel equalization is a process of compensating for the effects of changing conditions and disturbances on the signal transmission channel. This process typically employs filters that remove amplitude and phase distortions resulting from frequency dependent, time variable characteristics of the transmission channel, thereby improving symbol decision capability.
Carrier recovery is a process by which a received RF signal, after being converted to a lower intermediate frequency passband (typically near baseband), is frequency shifted to baseband to permit recovery of the modulating baseband information. A small pilot signal at the suppressed carrier frequency is added to the transmitted signal 1 to assist in achieving carrier lock at the VSB receiver 21. The demodulation function performed by demodulator 22 is accomplished in response to the reference pilot carrier contained in signal 1. Unit 22 produces as an output a demodulated symbol datastream 4.
ADC 19 oversamples the input 10.76 Million Symbols per second VSB symbol datastream 2 with a 21.52 MHz sampling clock (twice the received symbol rate), thereby providing an oversampled 21.52 Msamples/sec datastream with two samples per symbol. The advantage of using a two sample per symbol scheme as compared to one sample per symbol is the ability to use symbol timing recovery schemes such as the Gardner symbol timing recover method.
Interconnected to ADC 19 and demodulator 22 is a segment sync and symbol clock recovery network 24. The network 24 detects and separates from random noise the repetitive data segment sync components of each data frame. The segment sync signals 6 are used to regenerate a properly phased 21.52 MHz clock which is used to control the datastream symbol sampling performed by ADC 19. A DC compensator 26 uses an adaptive tracking circuit to remove from the demodulated VSB signal 4 a DC offset component present in the pilot signal. Field sync detector 28 detects the field sync component by comparing every received data segment with an ideal field reference signal stored in the memory of the receiver 21. The field sync detector 28 also provides a training signal to channel equalizer 34. NTSC interference detection and filtering are performed by unit 5, an example of which is disclosed in U.S. Pat. No. 5,512,957, entitled METHOD AND APPARATUS FOR COMBATING CO-CHANNEL NTSC INTERFERENCE FOR DIGITAL TV TRANSMISSION, issued on Apr. 30, 1996, to Hulyalkar. Afterwards, the signal 7 is adaptively equalized by channel equalizer 34 which may operate in a combination of blind, training and decision directed modes. An example of an adaptive channel equalizer is disclosed in U.S. Pat. No. 6,490,007, entitled ADAPTIVE CHANNEL EQUALIZER, issued on Dec. 3, 2002 to Bouillet et al. The output datastream from NTSC filter 5 is converted to a one sample/symbol (10.76 Msymbol/sec) datastream prior to reaching equalizer 34.
Equalizer 34 corrects channel distortions, but phase noise randomly rotates the symbol constellation. Phase tracking network 36 removes the residual phase and gain noise in the output signal received from equalizer 34, including phase noise which has not been removed by the preceding carrier recovery network 22 in response to the pilot signal. The phase corrected output signal 9 of tracking network 36 is then trellis decoded by unit 25, deinterleaved by unit 24, Reed-Solomon error corrected by unit 23 and descrambled by unit 27. The final step is to forward the decoded datastream 10 to audio, video and display processors 50.
In the receiver 21, the output signal 11 of the Reed-Solomon decoder 23 includes data sent in packets for subsequent processing by the audio, visual and display processors 50. The data is accompanied by a data framing signal, a clock signal, and an error signal that indicates whether or not the decoder 23 detected an uncorrectable error in the data packet. Typically, the decoder unit 23 generates the error signal via circuitry within the decoder 23 dedicated to this purpose. However, if the error generating hardware does not work correctly, additional expense must be incurred by incorporating hardware in subsequent stages that will assist in the generation of the error detection signal. Ideally, a software based solution is needed which will eliminate the need for including redundant error detection circuitry in an HDTV receiver.